1. Field of the Invention.
This invention relates to a method for producing high-quality silica and fluorine-containing coproducts.
2. Description of the Prior Art
Phosphate rock normally contains appreciable quantities of fluorine compounds and of silicon compounds. When such phosphate rock is used to produce wet process phosphoric acid, much of the fluorine and silicon content of the phosphate rock is converted to fluosilicic acid, H.sub.2 SiF.sub.6. The quantity of fluosilicic acid generated at individual wet process phosphoric acid plants can be quite large, e.g. a plant producing 800,000 tons of P.sub.2 O.sub.5 /year as wet process phosphoric acid could generate about 30,000 tons per year of fluosilicic acid. The fluosilicic acid produced is considered an undesirable by-product, since there is typically little market for it and, additionally, the fluosilicic acid can cause atmospheric and stream pollution problems. The industry has long sought, with limited success, a worthwhile use for the fluosilicic acid.
One use that has been sought for fluosilicic acid is the production of high grade silica by reacting the fluosilicic acid with ammonia compounds. U.S. Pat. No. 3,271,107 is an example of a process producing silica via the ammonia-fluosilicic acid reaction. Another use sought for the fluosilicic acid is to utilize fluorine values contained in the fluosilicic acid to produce fluorine-containing products such as ammonium fluoride and metal fluorides. U.S. Pat. No. 4,026,997 is an example of a process producing ammonium fluoride from fluosilicic acid. U.S. Pat. No. 4,264,563 is an example of a process producing a metal fluoride from fluosilicic acid. It does not appear that processes producing either silica exclusively, or fluorine-containing products exclusively, from fluosilicic acid can be commercially successful at today's prices for these commodities. It appears that a process must recover both a silica product and a fluorine product or products to be economically successful. The processes described in U.S. Pat. Nos. 4,308,244 and 3,549,317 recover silica as well as metal fluorides from fluosilicic acid.
Despite the availability of the above processes for recovering silica and fluorine products, only small amounts of the fluosilicic acid available are being processed. Even though there has long been a need in the industry for an economical use for the fluosilicic acid, the presently available processes are obviously not economically effective, since so little fluosilicic acid is being processed. A major reason that has made some of the existing processes uneconomical is that the silica produced contained excessive impurities, rendering it unsuitable for most industrial applications.
Obviously a need exists for an economical process for producing high quality silica and fluorine products from fluosilicic acid.
Thus, an object of this invention is to provide an economical use for byproduct fluosilicic acid produced in wet process phosphoric acid plants. Another object of this invention is to provide a method of economically extracting valuable products from fluosilicic acid. Still another object of this invention is to produce high-grade silica and fluorine-containing products from fluosilicic acid. Yet another object of the invention is to produce, from fluosilicic acid, silica and fluorine-containing products with low levels of impurities.
The invented process described herein is directed toward accomplishing the above objects.